African Pilot - April 2006
Slick 360 - Text by Glen Dell and Francois Jordaan / Photos by Athol Franz.
Flying the Slick 360 by Glen Dell.
There are numerous similarities throughout motor sport for the creation of the ideal vehicle. Whether it be a racing car, motorcycle, boat or plane one wishes to achieve the ultimate balance of power, weight and maneuverability. This was certainly the criteria for the development of the Slick 360.
The size of the aircraft was determined by the desire to build an aircraft that would be competitive at Unlimited level but be powered by a four cylinder engine. An aircraft that is very small, like the Pitts S1S or One Design, is difficult to judge accurately. A bigger aircraft incurs weight and drag penalties which ultimately require more power to maintain acceptable power to weight ratios.
The addition of a more powerful and heavier engine creates a greater problem than increased mass however. In order to maintain the Centre of Gravity in the ideal position the cockpit (and occasionally batteries etc.) needs to be moved far back behind the spar so that the weight of the pilot balances the engine. As various components and the pilot are moved further away from the C of G, greater control forces are required to overcome the inertia to start a maneuver and to stop the momentum thereafter. Invariably this will result in greater control forces and difficulty in achieving good scores in a sequence. In some cases certain maneuvers simply cannot be flown in an aircraft that has mass which is dispersed too far from the C of G.
An aircraft that has a long fuselage will also be difficult to flick roll, stall turn and do rolling circles. In competition aerobatics the aircraft must make the flying as easy as possible for the pilot while he or she concentrates on the sequence and positioning.
Flying the Slick is a pleasure indeed! It is simply so easy to push the 475kg plane out of the hangar, do the pre-flight and get into the cockpit with its forward opening canopy. The five point Hooker harness allows the pilot to strap himself in as tightly as he wishes. Instrument layout is conventional and starting is made easy by the electronic ignition.
The Slick has a 240HP AEIO360 engine. The power is achieved by increasing the compression ratio to ten to one, electronic ignition, cold air induction and other clever modifications undertaken by AeroSport Power in Canada. Taxing is safe and simple as the pilot can quite easily see over the nose. Power and pre-take off checks are the same as those for most light aircraft. Once lined up on the runway it is important to lock the tailwheel to assist in keeping the aircraft straight, particularly during landing. Power can be applied as quickly as desired and the tail lifted immediately. Acceleration is good with the plane becoming airborne at about 65 mph. A climb at 78 mph will give a rate of climb of around 1800 feet per minute at 8000 feet density altitude.
Aerobatics are easy to do – that was the main aim of building the aircraft. The airframe is incredibly rigid as one would expect from a (mostly) Carbon Fiber design. Control is very direct with maneuvers being very easy to start and stop. Flick rolls are particularly fun to do as the rate of rotation is quite phenomenal.
Pre-landing checks are conventional save for checking once again that the tailwheel is locked. Initial approach at 90 mph will allow a nose attitude that is low enough to give a good view of the runway. Over the fence at 80, the threshold at 75 and touchdown at seventy in the three point attitude. The locked tailwheel will keep the aircraft straight with very little rudder input required.
The success of the Slick is already well proven by the rave reviews of pilots that have flown it and possibly more importantly, competition results. With six planned to be flying in South Africa by the end of 2006 this little gem seems certain to set the SA aerobatic standard for the future.
Slick 360 Structural Integrity - by Francois Jordan
The pitch of the television commentator’s voice reaches hysteria level. “…coming through gate 3 on knife edge! Quick roll to the opposite side, entering a tight turn. The g-meter registers 7.9! Wow! Roll to level and he is perfectly aligned for gate 4…”
This is a typical scenario that the Slick 360 is designed and built for. During display or competition flying, or practise for that matter, the aerobatic pilot has enough on his mind not to have to be concerned about the integrity of his aircraft. For this reason we designed the Slick 360 to have superior performance, excellent handling qualities and unquestionable structural integrity.
Any aircraft structure should be designed for an optimum balance between required strength and minimal mass. Too strong implies too heavy, and in all things aeronautical weight is the enemy! Save some weight and the loads decrease, resulting in more possible weight saving. This is an example of the “weight spiral” going downwards! The secret is to build every part just strong enough.
How is “strong enough” defined? The designer adopts an airworthiness standard (in our case the Federal Aviation Regulations Part 23, or FAR-23 of the United States of America), which defines a minimum limit load factor for the relevant category of aircraft (normal, utility or acrobatic category). Load factor is simply the number of times the load experienced by the structure during a manoeuvre is more than that experienced during unaccelerated level flight. Load factor is often expressed in terms of “g’s”. Straight and level flight is flight at 1 ”g”, and a co-ordinated turn with a 60 degree bank angle requires 2 “g’s”. FAR 23 requires a minimum positive load factor of +6 “g” and a negative load factor of –3 “g” for aerobatic category aircraft. To accommodate competition aerobatics we chose limit load factors for our Slick 360 aircraft of + 10 ”g” and – 10 “g”. It is unlikely that the pilot could exceed these values, but we still apply a factor of safety of 2.0 rather than 1.5 as required by FAR-23. This means that no part of the Slick aircraft should fail before it reaches +20 or –20 “g” loads.
Structural integrity is often regarded as the strength of the wings only. Whilst wing strength is certainly very important, every other structural component needs to be strong enough as well. To demonstrate structural integrity the major components of the first all-composite Slick airframe were subjected to 15 “g” loads in static test rigs.
Initial tests on the fuselage indicated an area on each side, just aft of the firewall where the carbon fibre skin started buckling at about 8 “g” and the bond to the firewall flange failed at about 11 “g”. To rectify this the skin area where buckling occurred was fitted with a “sandwich panel” by incorporating an 8 mm PVC foam core and two more plies of carbon fibre. A subsequent test was performed to 15 “g” without any signs of buckling or impending failure. In fact the test rig failed at this load!
Investigation of the wreckage of a new all composite aerobatic aircraft which experienced a wing failure at less than half of its design limit load indicated a manufacturing flaw in the wing main spar. This went undetected during manufacture and subsequent inspection. To eliminate risks of this nature, AERO-CAM, the company manufacturing the Slick 360, will test every single wing in a static loading test rig to +11 “g” before integrating it with the fuselage. In this test rig, called a whiffletree, the aerodynamic loads on the wing are simulated through a system of beams and links connected to a single hydraulic actuator for each wing. The load applied to the wing is measured by means of a calibrated load cell. The wing needs to sustain the test load (11 “g” in our case) for at least 3 seconds. At the same time the aileron movements must be shown to be free and easy. After the test no signs of residual deflection or structural failure (cracks, delaminations etc) should be evident. This test establishes the structural integrity of each specific Slick 360 wing before it is flown for the first time.
In order to prove the design up to limit load one Slick 360 wing was tested to +15 “g”. Again the test rig failed and had to be strengthened twice before this load could be demonstrated. It is planned for some time in the future to test a wing to failure, which should occur at 20 “g”. This will finally verify our design process. In the meantime we are confident the wing can sustain loads 50% in excess of the design limit load.
Another factor playing a role in structural integrity is flutter. Flutter occurs in an aircraft component when the frequency of an aerodynamically excited vibration coincides with an undamped natural frequency of that component. This phenomenon generally occurs at high speed and is capable of destroying an airframe in a fraction of a second, usually with catastrophic results.
To determine the flutter characteristics of our Slick 360 the completed aircraft was subjected to Ground Vibration Testing by a group of specialists at the Council for Scientific and Industrial Research (CSIR) under leadership of Dr Louw van Zyl, an internationally recognised expert in this field. In this test procedure a number of accelerometers are fitted to the airframe to measure the response to vibration input over a range of frequencies. Once the airframe vibration modes, frequencies and damping are determined in this way a computer simulation of the aerodynamically excited vibrations and the airframe response can predict flutter which may occur in flight.
The Slick all composite airframe is constructed using mostly carbon fibre material. This provides superior rigidity of the structure, which is beneficial from a flutter standpoint. The results of ground vibration testing indicated that no flutter modes in the Slick aircraft reached zero damping (flutter indication) up to flying speeds of 500 knots! Since our maximum dive speed is limited to less than half that speed, it is probably safe to conclude that Slick is flutter free throughout its operating range.
The most common flutter occurrence in an airframe is probably aileron induced wing flutter. For this reason ailerons need to be balanced with their centre of gravity coinciding with, or close to, the hinge line. This in turn requires a substantial amount of lead in the aileron leading edge. On the basis of ground vibration tests it was determined that the aileron (and elevator) mass balance requirements could be somewhat relaxed, resulting in a massive 14 kg weight saving!
To verify the flutter characteristics of the aircraft, a series of in flight flutter measurements were made. Two aerodynamic “shakers” were fitted to the wing tips, which could induce vibrations through a frequency range during flight. Again accelerometers were fitted at certain points on the airframe and the response to vibration input recorded for a range of flying speeds up to the maximum dive speed of the aircraft. At the time of writing the accumulated data were being processed, but initial indications are that the flutter free results of the ground vibration tests were confirmed.
It would be difficult to remove balance mass from the ailerons after they were closed, so it was decided to build a new set for the aircraft. At the same time it was decided to move the hinge line a little aft, to reduce the lateral stick loads. Subsequent test flights indicated an aileron “snatch” tendency at high speed. This is probably due to moving the hinge line without adjusting the aileron shape ahead of the hinge line. It has been agreed with the CSIR that we can perform wind tunnel tests with three different aileron shapes. Our purpose is to optimise the aileron configuration to provide adequate roll rate and well harmonised controls. The tunnel models are being prepared at the time of writing and it is hoped to have the results within a few weeks.
Our goal to produce the world’s best aerobatic aircraft is now within grasp. We have constructed assembly jigs for all major components and production of the next batch of three aircraft is well under way.